1. Field of Invention
The present invention relates to a method for the removal and recovery of elemental sulfur from gas streams that contain sulfur gases and are highly reducing due to the presence of large quantities of hydrogen and carbon monoxide.
2. Discussion of the Background
Gasification of heavy carbonaceous feeds (e.g. coal, petroleum coke, petroleum resids, biomass and others) produces a synthesis gas (syngas) containing greater than 30 volume percent and as much as 90 volume percent of reducing gases, hydrogen and carbon monoxide (H2 and CO), depending on the different gasification processes employed such as KRW, Texaco, Shell etc. The sulfur bound to these feeds gasifies to hydrogen sulfide (H2S) that remains in the syngas as a major contaminant upon gasification. H2S must be removed and disposed before the syngas can be further utilized to produce electricity in a gas turbine or liquid fuels and chemicals (for example using Fischer-Tropsch synthesis). Presently, removal of H2S involves a solvent-based process (e.g. using amine solutions). The amine-based process involves a number of steps and is highly equipment intensive. The raw syngas is first contacted with the amine solution using a gas-liquid scrubber. The spent amine is then regenerated using steam and the regeneration off-gas containing H2S is sent to a Claus plant. The Claus plant consists of an H2S burner, which oxidizes ⅓ of the H2S to SO2. The H2S—SO2 mixture is then sent through a series of up to three catalytic reactor stages with inter-stage coolers to carry out the Claus reaction (2H2S+SO2=2H2O+⅜S8) and recover sulfur. The tail gas from the last reactor still contains about 2% of this inlet sulfur. It is sent to a tail gas treatment plant such as Shell-Claus Off-Gas Treating (SCOT) plant, which is as big as ½ the size of the Claus plant. As is seen, the solvent route to H2S removal is quite cumbersome.
Another class of solvent-based processes involves liquid-phase redox catalysis. In these processes (such as Stretford, Locat etc.), the H2S is reacted with a liquid-phase catalyst containing iron and/or vanadium chelates. The reacted catalyst is then oxidized in a separate reactor to directly produce elemental sulfur. Although this class of processes is attractive in that they avoid the use of a Claus plant, the elemental sulfur produced is contaminated with the catalyst constituents. These processes have generally been limited to small plants typically less than 2 tonnes/day sulfur and suffer from numerous mechanical and process chemistry problems.
Yet another class of solvent-based processes, the so called “liquid-Claus” processes involves the reaction of H2S and SO2 in a liquid solvent (e.g., U.S. Pat. No. 4,107,269), such as metacresol, benzyl alcohol, 2-phenoxy ethanol, diethylene glycol, ethyl ether etc. with a soluble catalyst such as potassium benzoate. Again, these processes are attractive in that they avoid the use of a Claus plant. However, solvent losses would result and the sulfur would be contaminated with the solvent and/or catalyst. Consequently, the authors are not aware of any commercial installation employing this approach.
Alternative dry regenerable zinc-based sorbent processes are under development. In these processes the H2S is reacted with zinc oxide (ZnO) in an absorbing reactor at temperatures above 250° C. The spent sorbent is regenerated in a separate reactor using air to produce a dilute SO2-containing off-gas. The SO2 containing off-gas can be subjected to sulfur recovery using a number of alternate processes such as Wellman-Lord/Augmented Claus or Direct Sulfur Recovery Process (DSRP), which catalytically reduces the SO2 to elemental sulfur (U.S. Pat. Nos. 5,366,717 and 5,798,088). These alternative process schemes are also quite complex utilizing three or more reactors.
In all commercial processes conceived to date, removal of H2S from the highly reducing raw syngas involves a first step to absorb or adsorb the sulfur gases using a liquid solvent, metal chelate solution or dry sorbent. Then another step is needed to free up this sulfur either as another gas stream containing sulfur that must be further extensively treated or as contaminated elemental sulfur that must be cleaned before disposal. These processes either involve highly equipment intensive Claus and off-gas treating plants or produce contaminated sulfur rather than Claus-quality (equivalent to sublimated-quality) sulfur.
Thus conventional and developmental methods for removing H2S and recovering elemental sulfur from highly reducing syngas streams are complex multistage processes. There is therefore a need for a simple, direct, and continuous process for removing sulfur gases and recovering Claus-quality elemental sulfur from highly reducing syngas streams containing 30 to 90 volume % reducing gas (H2+CO) components.